Microspheres are an example of a drug delivery system that has been evaluated extensively in several therapeutic fields. They are essentially solid particles with 1 to 500 μm in diameter which can both target their drug cargo by physical trapping in blood vessels (chemoembolisation) and sustain the action of a therapeutic agent through controlled release. Microspheres can be made from a broad range of materials, including proteins, polysaccharides, polyesters and lipids by a variety of different techniques (emulsification, heat stabilisation, coacervation and phase inversion technology). Microspheres are monolithic structures, solid throughout, and distinguishable from more fluid and flexible vesicular systems such as liposomes. They are normally 1-500 μm in diameter and fall between granules (>100 μm) and microparticles (<1 μm). They distinguish from microcapsules for their internal structure, being a homogeneous matrix rather than a vesicular form. Microspheres can be produced from a number of different biocompatible biodegradable materials such as protein (albumin and gelatin) (Biopharm. Drug. Dispos. (1985) 6 pp. 91-104 and Intern. J. Pharm. (1987) 35 pp. 177-179), polyesters (glycolide and lactide) (J. Microencap. (1986) 3 pp. 181-193 and Drug Dev. Ind. Pharm. (1990) 16 pp. 2353-2367), polysaccharides (starch, ethyl cellulose, alginate and chitosan) (Drug Dev. Ind. Pharm. (1996) 22 pp. 457-463 and J. Contrl. Rel. (1997) 43 pp. 65-74), ion exchange resins (J. Contrl. Rel. (1989) 8 pp. 251-257) and lipids (Adv. Drug Deliv. Rev. (1996) 20 pp. 209-219).
Until now, many approaches have been developed to form microspheres whilst simultaneously encapsulating the drug, including such diverse techniques as:
                chemical stabilisation (Biopharm. Drug. Dispos. (1985) 6 pp. 91-104);        heat stabilisation (Experientia (1983) 39 pp. 913-916);        multiple emulsion solvent evaporation (J. Contr. Rel. (1994) 28 pp. 121-129);        multiple emulsion solvent extraction (J. Contr. Rel. (1997) 43 pp. 261-272);        coacervation (Cancer Res. (1993) 53 pp. 5841-5844);        phase inversion nanoencapsulation (PIN) (Nature (1997) 386 pp. 410-414);        spray drying (Pharm. Sci. (1997) 86 pp. 603-607).        
Occasionally a drug is added to or complexed onto microspheres after particle formation. Selection of the matrix material and method of preparation are critical in defining overall performance.
The choice will depend on several factors:                size of microspheres required;        inherent properties of the drug, for example, aqueous solubility and stability;        surface characteristics of particles, such as permeability and charge;        degree of biodegradability and biocompatibility;        drug release profile desired.The rate with which the drug is released from microspheres is dependent on three main factors:        solubility of the encapsulated drug and diffusion processes;        rate of particle erosion and biodegradation;        interaction between the drug and the particle matrix leading to immobilisation.Polymeric microparticle are usually prepared by techniques such as single/double emulsion-solvent evaporation, coacervation and spray drying.        
These techniques however show some drawbacks: in the solvent evaporation method, high quantity of chlorinated organic solvents are normally used and controlled operative conditions can be rarely achieved; moreover in the case of peptide and proteins the solvents used can denature the structure and lead to a loss of potency. In the O/W single and double emulsion it has been also reported that the accumulation of amphiphilic molecules (i.e. proteins) at organic/aqueous interface layer could cause drug aggregation and precipitation. (Pharmaceuticals Dosage Forms: Disperse systems 2nd Edition. Marcel Dekker Inc. (1998) pp. 163-193).
Spray-drying is a technique in which the polymer and the drug, solubilized or suspended in a medium, are atomized through a nozzle in a chamber where the solvent is forced to evaporate by the effect of a relatively high temperature and the microparticles are collected in powder form at the end of the process. By such evaporation technique the matrices obtained are normally quite porous, leading to a poor drug encapsulation within the matrix resulting in a fast release and in a large initial burst effect. Moreover the air/liquid interface formed during the preparation enhance molecule aggregation (especially for proteins) at the surface. (Mumenthaler M. et al. Pharm. Res. 11 (1994), No. 1).
Therefore the cryogenic micronization can be envisioned as alternative manufacturing method for obtaining microparticles of lipid material, that could lead to remarkable advantages, both in terms of peptide/protein stability and in terms of reducing the burst effect. Moreover, drug release profile can be modulated by obtaining a defined physical state of the lipid, having said lipids various crystalline states (such as polymorphic states).
In the examined prior art, some examples of lipid microparticles have already been described for industrial application in the field of drug release. W. Steber et al. (American Cyanamid Corporation, EP 257368) describe a microsphere composition, containing from 30 to 95% of fats or waxes and about 2 to 70% of a biologically active substance, where the lipid component contains a glyceril tristerate content from 55 to 79%. M. W. Fountain et al. from The Liposome Company, U.S. Pat. No. 4,610,868) claim lipid matrix carriers comprising a hydrophobic compound, an amphipathic compound and a bioactive agent, combined in the form of globular structure, having a diameter from 500 nm to 100 μm. Said carrier is obtained by emulsifying the components and injecting the emulsion into an organic solvent. H. Augart (Warner Lambert Company, U.S. Pat. No. 4,483,847) described a composition for the delivery of drugs, comprising both high and low melting lipids, that after melting, mixing and cooling are granulated for the production of tablets. P. Orsolini et al., (Debiopharm, U.S. Pat. No. 5,192,741) describe a process comprising a cryogenic grinding step, for preparing a pharmaceutical composition containing polylactide, copolymer of lactic and glycolic acid and peptides. Microparticles are obtained by dissolving/dispersing said polymers and the bioactive agent into an organic solvent, removing the solvent while shaping the solid residue.
It is therefore an objective of the present invention to provide lipid microparticles with sustained release and especially a low “burst effect”.